Nursing is only one sphere of CNS practice. To give the least complex explanation, I would Google 'CNS 3 spheres of influence'-patient, nurse, and system. This will explain that as one of the 4 types of APRNS, we diagnose, treat and care for a population of patients from wellness to acute illness inclusive of health promotion and disease prevention, prescribe, educate patients and families, and act as mentors to nursing staff, and much more.
This is not to say that some CNSs have much more of a clinical role and interest and practice more in the patient sphere, while others practice more in the organization/nursing sphere and act as change agents and mentors. This is likely the 'non diagnosing and prescribing' role you describe (which in some states without rx auth is what some CNSs do). But I assure you CNS practice is alive and well in all permutations of the 3 spheres where allowable.
In response to your stance of presuming all other states are like your own, I would love to presume my state was like some others and afforded full practice authority for all APRNs, but alas it is not so. I honestly find educating others about the CNS role is one of the most challenging parts of becoming one, because different areas of the US are so divergent at this time, as we discussed earlier about some states only having recently accepted 'CNS' as a protected title that requires an MSN in CNS studies to utilize. May the consensus mode help us all!